From the prior art a drum made out of steel sheet is blown. Such a drum has a substantially cylindrical shape with a closing cover at each end of the cylinder. Further the cylindrical surface of the drum is provided with corrugations in the circumferential direction. Each corrugation typically consists of an elevated region that is relatively elevated relative to the average cylinder radius and a deepened region that is relatively deepened relative to the average cylinder radius.
Currently, steel drums from the prior art are manufactured from a flat rectangular steel sheet, cut to size, that is rounded in one direction to form a cylindrical shape. The free ends of the steel sheet are brought together and are seamed or welded in the same process.
Next, the cylindrical shape is processed in a corrugator that produces the corrugations in the sheet in such a way that the corrugations each form an circular shape around the main axis of the cylinder.
The closing covers are typically attached to the steel cylinder by applying a folded seam. The folded seam may additionally be welded by electrical or radiative (i.e. laser) source. In case of an open head drum a cover can be arranged on the opening and fastened by means of a closing ring.
The corrugations in the cylindrical surface may be created by a rolling process or by a mechanical deformation of the cylindrical shape within a mould by exerting pressure on the wall of the cylindrical shape. Typically, two main beads are formed by exerting pressure. Between the two main beads a flat surface remains present while between each main bead and the respective closing end additional corrugations may be formed by rolling. The additional corrugations if present have smaller size than the main beads.
The corrugations provide a structural reinforcement that strengthens the drum against under-pressure.
Typically, the steel drum is divided by two main corrugations into a bottom cylindrical part, a center cylindrical part and a top cylindrical part, in which the center cylindrical part is substantially flat.
The bottom and top cylindrical parts may comprise additional corrugations that have a smaller depth (i.e., level difference between the elevated and deepened part) in comparison to the main corrugations.
Prior art steel drums are manufactured with various sizes and volumes. For use in container transport, steel drums have a cylindrical design and have a standardized diameter for optimal stacking in ISO standard containers. As an example, a typical internal diameter of such standardized steel drums according to ISO 15750 is about 570 mm (external diameter 585 mm), with a height of 850 mm, with a volume of nominal 216 liters.
A typical example of a steel drum for use in container transport is the well-known W-style bead type steel drum as described e.g. in U.S. Pat. No. 5,950,472, which provides a combination of a sufficient dynamic compression strength (during stacking) and a sufficient vacuum collapse strength.
To reduce the manufacturing costs per drum, there is a tendency to use steel sheet as thin as possible since less raw material and less energy costs per drum are needed. Additionally, using thinner steel sheet as raw material in the manufacturing of the drum will result in a lower weight per drum and lower energy costs of transport, since the ratio between the dead weight of the drum and the weight of the contents can be reduced further.
However an adverse effect may be that, using a thinner steel sheet, in particular 0.8 mm and thinner, will have a detrimental effect on the mechanical strength of the steel drum.
It is therefore an objective of the present invention, to provide a steel drum which has a relatively reduced weight while the mechanical performance is the same or better than for prior art steel drums.